Advances in composite technology have had a marked impact on product design and engineering and, ultimately, manufacture. Early methods involving hand lay-up of fibrous materials and sheets with the subsequent impregnation of the laid up materials and, subsequently, the laying up of pre-impregnated fibrous sheets and mats followed by compression forming and curing saw rapid adoption and exploitation of these composite materials and technologies in many fields. However, while useful for many applications, the slow methodical build-up of the layers of materials is very labor intensive, involves the use of hazardous chemicals and, more importantly, oftentimes very unstable materials and/or materials having limited working time. Thus, while a marked advance in the industry, their applications were still limited.
Subsequent advances in composite materials and technology led to continuous manufacturing techniques. Most notably, filament winding where a continuous tow of certain fibrous materials were pulled though a bath of a curable material to impregnate the same with the curable material, wound about a mandrel to form the desired part, preferably with some measure of immediate cure to attain a green state so as to maintain its shape and fiber placement and subsequently fully cured. However, these operations were very slow and time consuming owing to the narrow width of the tow. More importantly, these processes were very capital intensive as the whole of the operation, from preparation of the curable composition to wetting of the tow of fiber material to the formation of the part itself, all had to be performed in the same room.
Continued advances in both manufacturing and materials technology led to the ability to prepare rolls of prepreg materials, especially epoxy impregnated fibrous sheets, wherein the base sheet material was most typically formed of unidirectional, parallel fibers running the length of the roll. These rolls were typically of sufficient length, like the rolls of tow in filament winding applications, that continuous manufacturing methods could be developed with the prepreg materials being formed at the prepreg manufacturer, cooled to prevent premature curing, shipped to the site of use and warmed and subsequently used to produce the desired products. No longer was it necessary for the ultimate product manufacturer to invest capital and overhead or employee technical personnel to make the prepreg materials. Cost advantages and a focus on centered expertise (e.g., prepreg manufactures concentrated on the chemistry and manufacture of the prepreg and the product manufacturers concentrated on the layup process) proved beneficial all around.
Early on, prepreg strips or sheet materials were manufactured in standard widths that were used to make the commercial products. This was acceptable in the early going as many applications had very similar demands and requirements, e.g., baseball bats, golf clubs, hockey sticks, lacrosse sticks and the like can all be made with generally the same width of slit tape, largely because the demands are similar and the need for differentiation less. Even if not optimal, these stock tapes were used nonetheless as making many different widths, especially making custom widths, was cost prohibitive. The capital requirements are quite large and incapable of supporting a large differentiation of products. Consequently, the full adoption of this technology in higher demanding applications was limited if the width of the tapes needed were inconsistent with the widths that were available.
More recently, technology has evolved and new expertise and processing capabilities have been developed whereby a broad array of tape widths were made possible through the slitting of stock master rolls. Wide tapes could now be made for applications having large planar or curved planar surfaces while narrow width tapes could be made for more intricate or curved parts. Though this technology allowed for the use of prepreg tapes in the manufacture of many different products whose demands, especially physical demands, required specific properties which are affected by, in part, the width of the prepreg tape, the expansion of the use of prepreg tapes into the production of aircraft parts has been one of the, if not the, major driver for recent technology innovation and advancement. While one might think that the demands for aircraft production would be fairly constant across the field of aircraft components, nothing could be further from reality. Each component must endure its own, most often unique, environment and its respective physical demands and stresses. For example a tail fin will have marked different requirements than a wing or a fuselage portion. Combined with the tight tolerances of aircraft parts, it has become more and more apparent that a plethora of widths of slit tape are needed with differences in width for slit tape from one application to the another being on the order of just fractions of an inch, and minor fractions at that. Tolerances are orders of magnitude smaller, with tolerance requirements being on the order of thousandths of an inch. Further confounding the process in aircraft production is the finding that certain parts require slit tapes off two or more different widths. Thus, custom slitting has been a major achievement in the use of slit tow in composite structure manufacture.
Notwithstanding its benefits, the adaptation of slit tape, especially for aircraft production, is not without its shortcomings. Most notably, owing to the high cost of the machinery and apparatus to make master rolls, master rolls still tend to be produced in standard widths. With custom width slitting of these master rolls, the likelihood that the width of the master roll will be a whole multiple of the width of the slit tape desired is rather low. Consequently, the ability to slit to custom widths comes with the disadvantage of an increase in processing waste, a costly event given the high costs of prepreg materials. The only alternative is to adjust the slit tape widths to allow for full consumption of the stock roll; however, this may mean that the optimal width for the given end use part may not be used. While this may not be an issue for an internal, non-structural component of an aircraft, it is simply not tolerable for a structural member or a fuselage application.
Additionally, as noted, higher demand applications, especially in aircraft production, are now found to require the simultaneous, alternating and/or sequential application or placement of slit tapes of two or more widths. Most often this involves the use of a wide slit tape, one that must or is preferably wound on a spool or reel whereby the resultant stock material looks more like a disc or pancake and is often called a pancake coil. The second slit tape required is quite often, if not most often, at the other end of the width scale, being an inch or less, which widths are not capable of being, or are preferably not, wound on a spool but must be wound about a spindle in a spiraling or helix type pattern (transverse winding), much like a package of twine which is typically wound on a dowel or the like.
Given the different demands and requirements in producing narrow and wide width slit tapes, until the present teachings, it has been the standard in the industry that the two different slit tapes be formed on two different apparatus, each configured for the specific tape width desired. Alternatively, they have been produced sequentially using, for the most part, the same equipment but changing out the slitter element to slit the different widths as well as the winding apparatus. The first option of two dedicated lines is the most convenient and has the least impact on processing downtime, but has the added cost of capital and operation expense in order to install, support and operate two complete lines. The second option is less costly, especially in terms of capital commitment and space requirements, but has significant cost impact in terms of production time and, at the time of change over, manpower. Whereas the process employing two separate apparatus can run continuously, with minimal interruption but to change the master roll and off load the spooled/spindled slit tape; the latter requires considerable downtime to switch out the slitting element for the second tape width as well as to switch out the winding station and apparatus and reconnect the remainder of the slitting apparatus to the new winding station and apparatus, prime the system and get it up and running for the different product. All of this adds considerable costs to what is already a very expensive process and product as well as raises concerns for the time such materials remain on the processing floor, exposed to ambient conditions, particularly given their instability and need for storage under significantly reduced temperatures.
Additionally, although less of a concern in non-aerospace applications, another factor that comes into play with applications requiring two or more widths of slit tape is the fact that each width of slit tape comes from a different master roll which may be of the same lot or of an entirely different lot: perhaps even a master roll from a different prepreg manufacturing plant altogether. Sure all master rolls are manufactured to certain specifications, but there are tolerances in each specification and the specifications only address key attributes and/or properties of the master roll material. However, different master rolls, even from the same production lot, may experience different environments/conditions that have an impact on their ultimate performance, cure characteristics, properties and the like.
Again, in most applications this is not of concern, but in high tech aerospace and aircraft applications, it can be a considerable problem. Even an ever so slight difference in cure characteristics as between the wide tape and the narrow tape, especially at the interface of the two tapes, could have an adverse effect on the overall performance of that part. In this respect, consider the fact that the fuselage of the SST aircraft expanded several feet in length during supersonic flight. Materials compatibility and matched coefficients of thermal expansion were critical to the ultimate success of that aircraft. Thus, the use of parts where an incompatibility or weakness in the bond between one slit tape tow and another in the production of that part could play a role in the ultimate performance and life of that part.
While rigid product inventory control and management can reduce the likelihood that two disparate slit tapes will be used to make the same part, it may not always be possible. For example, the original quantity of stock master rolls may not allow for the proper ratio of tape widths. Furthermore, even if such management efforts were put in place, it still cannot address the fact that the two master rolls may yet have differences not readily apparent to the fabricator and/or inherent in the curable matrix owing to different handling during shipment and storage, e.g., one may have been left in the open longer than the other and heated to a different temperature than the other. As active, curable compositions, the properties of the matrix resin of the prepreg will change with time and temperature: a factor which, as noted above, comes into play the longer the rolls are left exposed to ambient temperatures as with extended or delayed slitting processing times.
Thus, there remains a need in the industry for a commercially cost-effective method for the production of wide and narrow width slit prepreg tape. Specifically, there remains a need for a simple, cost-effective, labor non-intensive method for the production of wide and narrow width prepreg slit tape with minimal waste and minimal processing time.
In following, there remains a need in the industry for an apparatus for use in a method that allows for a commercially cost-effective, particularly from a capital equipment perspective, method for the production of wide and narrow width slit prepreg tape.
There remains a need in the industry for a production method for wide and narrow slit prepreg tape which assures minimal, if any, differences in composition, treatment, handlings, etc. of the master stock rolls from which each is produced. Most especially, there is a need for slit tape production which does not require, or minimally requires, strict inventory management controls and oversight.